A method and means for making high molecular weight polyethylene sheets is described in applicant's copending application Ser. No. 860,145, filed Aug. 21, 1969. As was stated there, the term "high molecular weight polyethylene" means polyethylene or other polymer having a molecular weight of approximately 1,000,000 or greater, and have hot melt tensile strength three to six times greater than that of the usual high density extrusion resins in the molecular weight range of 200,000 to 250,000. By way of further comparison, the usual high density polyethylene extrusion resins in the molecular weight range of up to about 250,000 are about the consistency of molasses and are little more than self-supporting in their amorphous state as they leave the extruder. They can be easily fractured or ruptured in their hot melt or amorphous state by passing a gloved hand through the plastic, the glove being necessary only to protect one from the heat. On the other hand, the polymer resins in the molecular weight of 1,000,000 to 2,000,000 and above with which this invention is concerned exhibit unusual toughness or hot melt strength in the armophous state as it comes from the extruder. In fact, it will not fracture or rupture even when struck with a hammer while in an amorphous state. Such a material, since it is essentially incompressible exerts tremendous outward pressure on the walls of the extruder during the extrusion process. This fact imposes serious limits on the size of the extruded sheets since it has long been known that the outward pressure against the walls of the extruder increases with the cross-sectional size of the sheet and of the extruder.
It was known prior to this invention to transform high molecular weight polyethylene resin from powdered or granular form into rectangular panels or cylindrical billets by a process known in the art as compression molding. The transformation of the powdered or granular resin into a solid form was accomplished by coalescing the material under high pressure and heat above its crystalline melting point of 289.degree.F. and gradually cooling the material while maintaining the high pressure for a prolonged period of time. The amount of pressure and the length of time required to transform the granules into a solid state possessing the above optimum physical properties depends on the quantity of materials being processed, a larger quantity requiring a longer time at the same high pressure. High molecular weight polyethylene is virtually incompressible in its granular, crystalline, or solid form, and the high pressure is required to increase the density of the finished sheet to give it the desired physical properties.
As an example of the conventional molding technique of high molecular weight polyethylene, assume it is desired to produce a sheet of high molecular weight polyethylene about two inches thick by the conventional two-stage molding technique. In the first stage, a cold preform is made of the granular raw material, and the second stage comprises melt sintering of the preform. The preform is made by placing the granular raw material into a block form of the desired 2 -inch thickness and subjected to pressure of about 1400-2800 psi for 5 to 10 minutes. This results in a preform which with careful handling can be taken from the form. Normally, this cold-pressed block is loaded into the mold which is then placed in a press heated to 350.degree. to 400.degree.F. A time of two to three hours is necessary for the melting of these blocks without defects.
At the beginning of the melting, the block is loaded with a pressure of 300-700 psi on the preformed plastic. After the prescribed heating time of two to three hours, the block must be cooled for about one hour under pressure. During this cooling, a gradual increase of pressure is recommended. A pressure of 2500 psi or more must be attained on the plastic at the end of the cooling, to minimize voids on the interior and sunken spots on the surface of the blocks. Care should be taken so that the sides of the block do not cool faster than the center, and that pressure is uniformly distributed.
The necessity of carefully controlling the temperature of the mass of high molecular weight polyethylene during its transformation from powdered form to a solid slab has heretofore caused the use of extremely heavy molds capable of exerting the required high pressure and maintaining the desired heat for the necessary length of time to produce a satisfactory slab or shape. Production under such procedures is necessarily slow and costly, but the resulting end products fabricated from the sheets have found acceptance in several industries where the physical properties of high molecular weight polyethylene have proven advantageous. These physical properties, which have been briefly noted above, are more fully set forth in a publication of Hercules Powder Company, Incorporated, Wilmington, Del., entitled "HERCULES TECHNICAL DATA HIGH FAX N0. 30; PROPERTIES, USES, AND FABRICATION OF THE HIGH FAX 1900 SERIES; A Very High Molecular Weight, High Density Polyethylene With Unique Shock and Abrasion Resistance."
High molecular weight polyethylene has proven useful in the textile industry, for example, where it has been fashioned into pickers which drive a pound and a half sharpened steel-tipped shuttle back and forth across a loom more than 200 times a minute. High molecular weight polyethylene has gained a wide acceptance in the paper industry as a material for suction box covers because of its low coefficient of friction. The abrasion resistance of high molecular weight polyethylene lends to use as wear strips, its adaptability for this use also being enhanced by its low coefficient of friction. The discussion of the end uses of high molecular weight polyethylene is not in any way intended to be exhaustive but merely illustrative and for the purpose of pointing out the advantages of the present invention.
It is emphasized that the desirable physical characteristics of high molecular weight polyethylene are not attainable unless the high molecular weight polyethylene is transformed from powder form to a solid state or slab under carefully controlled conditions of pressure and temperature.
One advantage of the high molecular weight polyethylene products is their tremendous strength, although first attainable only by compression molding, may be more efficiently produced by a certain technique of ram extrusion. It has been previously known to extrude high molecular weight polyethylene from a granular form to a solid state, but all such previously known extrusions have been of a relatively small size because of the considerable length of time required to uniformly dissipate the heat from a large mass of high molecular weight polyethylene. An increase in the cross-sectional dimension of the extruded mass of high molecular weight polyethylene increases the thermal expansion of the high molecular weight polyethylene within the extruder. It is the tremendous thermal expansion inherent in high molecular weight polyethylene which enables it to be satisfactorily processed through an open-ended extruder. Otherwise, there would be no way to apply the necessary high pressures to the heated resin to produce a product having the desired physical properties.